1. Field of the Invention
This invention relates to the use of cyclophosphazene as a flame-retardant additive to the electrolyte of a lithium-ion battery, which material acts as a flame-retardant that reduces both the exothermic peaks and the maximum self-heat rate of the lithium-ion battery by increasing the thermostability of the electrolyte. Applications of this invention are in consumer batteries having carbonate-based electrolytes.
2. Description of Prior Art
Lithium-ion batteries possess high energy density compared to other secondary batteries. Small lithium-ion batteries having a capacity of 1300-1900 mAh are currently commercially available to power portable electronic devices such as camcorders, computers, cameras, etc. In addition, lithium-ion batteries are being developed as power sources for electric vehicles to provide longer drive ranges, higher accelerations and longer lifetimes. However, safety concerns have limited the full utilization of lithium-ion batteries in electric vehicle applications.
The primary challenge in designing an electric vehicle lithium-ion battery is its safety under abusive, as well as normal, operating conditions. Under abusive conditions, and occasionally even under normal conditions, lithium-ion cells undergo thermal runaway, producing exceedingly high temperatures, smoke, explosion and fire. In addition, under certain conditions, the flashpoint of the electrolyte can be exceeded and the lithium-ion battery can be overheated, thereby resulting in major safety problems.
Manufacturers employ external safety devices in small consumer lithium-ion batteries to overcome these problems. These devices include a smart charge control, poly-thermal switch which is responsive to a temperature rise in the cell, and a current path interrupter which is responsible to a rise in internal pressure. Use of these devices in batteries for electric vehicles is, however, not cost-effective. These critical limitations remain the main concern and drawback for scale-up of lithium-ion cells to the large sizes desirable for high-power application in electric vehicle propulsion systems. Thus, non-flammability of the electrolyte is an essential property for the safe design and operation of electric vehicle batteries.
One approach to this problem is the use of fire-retardant electrolyte compositions. U.S. Pat. No. 5,830,600 teaches a lithium battery comprising a fire-retardant electrolyte composition comprising a lithium salt dissolved in a solvent selected from the group consisting of a phosphate, a phospholane, a cyclophosphazene, a silane, a fluorinated carbonate, a fluorinated polyether, and mixtures thereof. That is, the electrolyte of the lithium battery of the ""600 patent is a phosphate-based electrolyte and, because phosphorous itself is a flame retardant material, it would appear as if the flame-retardant properties of this lithium battery are due to the presence of phorphorous in the electrolyte. The ""600 patent further indicates that electrolyte solvents, such a propylene carbonate, ethylene carbonate, diethyl carbonate and the like, although providing high conductivities in the presence of suitable lithium salts, are chemically unstable and, thus, unsafe.
It is one object of this invention to provide a flame-retardant lithium-ion battery comprising a non-aqueous solvent lithium-based electrolyte.
It is another object of this invention to provide a flame-retardant lithium-ion battery comprising a carbonate solvent lithium-based electrolyte.
These and other objects of this invention are addressed by a lithium-ion battery comprising an anode electrode, a cathode electrode, and a non-aqueous solvent lithium electrolyte, and a flame-retardant additive comprising at least one cyclophosphazene disposed in the non-aqueous solvent lithium electrolyte. In accordance with a preferred embodiment of this invention, the non-aqueous solvent lithium electrolyte comprises at least one carbonate solvent. As discussed hereinabove, U.S. Pat. No. 5,830,600 indicates that electrolyte solvents such as propylene carbonate, ethylene carbonate, diethyl carbonate and the like, which solvents form the basis of the flame-retardant lithium-ion battery of this invention, although providing high conductivities in the presence of suitable lithium salts, are chemically unstable and, thus, unsafe. Accordingly, it is indeed surprising and unexpected that the flame-retardant lithium-ion battery of this invention having a non-aqueous solvent lithium electrolyte comprising at least one carbonate solvent is stable and safe. As discussed in more detail hereinbelow, the flame-retardant additive incorporated into the non-aqueous solvent lithium electrolyte of the lithium-ion battery of this invention provides a considerable reduction in the exothermic peaks of the battery and the maximum self-heat rate of the battery is reduced by three times. Moreover, the addition of this flame-retardant additive greatly increases the thermostability of the electrolyte.